In Japan, the peak period during summer time is mid August that is called “Obon”. Most of Japanese go back to hometown and stay with family. All transportation will be busy from major cities at the beginning and back to major cities at the end. In 2015, the following dates will be busy.
Outgoing (from major cities) — August 12 and 13. Maybe August 7, 8 and 9 Incoming (to major cities( — August 15 and 16
If you travel during this period, I recommend you to avoid to make a trip to same direction as local tourist. It will be super busy.
And also, the public school is closed from July 20 to August 31 for summer break. This period is busier than usual too.
Both Cassiopeia and Hokutosei very popular. Many tourists have a plan to take one of these trains. However these overnight trains cannot be reserved online. The only way to reserve it from outside of Japan is calling Odekake net 078-341-7903 in Japanese and you have to pick up your ticket in JR West, Shikoku or Kyushu stations. You cannot pick up it in other areas, like Tokyo. Otherwise You need to be the reservation window at the station in Japan. Ticket is started to sell one month prior to the departure date. That means it may be sold out when you arrive in Japan. So you must have a back up plan when you try to make a reservation after arrival.